Handheld devices such as cell phones, personal digital assistants, and the like, commonly incorporate cameras, and typically include a cycling light source, such as an LED which acts as a flash for the camera.
The LED will be connected electrically to a circuit assembly. The circuit assembly may be of the type commonly referred to in the industry as a “printed circuit board” or of the type commonly referred to in the industry as a “flex circuit”. In either case, an electrical circuit is provided on a substrate of dielectric material.
In the case of a flex circuit, an electrical circuit, such as a copper circuit, is provided on the surface of a polymer material, such as polyimide or polyester, which functions as the dielectric layer. As the name suggests, these substrate materials are flexible and can even be provided as rolls of material. Such flex circuits may also be further supported by a stiffener, which can be a metal, a plastic, or other material, such as a pad of glass fiber laminate material, generally known as FR4 material, which provides more structural rigidity to the assembly so as to aid in the support and alignment of the LED light source within the handheld device.
In the case of printed circuit boards, those are conventionally manufactured from dielectric materials such as glass fiber laminates (sometimes known as FR4 boards), polytetrafluoroethylene, and like materials. On one of the surfaces of such board, or between layers of dielectric materials, are circuits, usually formed of copper. The circuits are commonly formed by photolithographic methods, sputtering, screen printing or the like (for circuits disposed between layers, the circuit is applied to the dielectric material before formation of the laminate). The light source can be disposed on the surface of the boards, in contact with the circuits on the surface. When a “flash” light source is employed, the light source generates substantial amounts of heat in a short period of time, that must be dissipated for the device to operate reliably and to its intended performance level. Indeed, for LED light sources, it is well-known that the cooler the junction temperature of the LED, the higher the instantaneous brightness of the LED is, and the decay of luminosity output slows (in other words, when the junction temperature of an LED is elevated, the LED exhibits lower instantaneous brightness and faster decay of luminosity over extended periods of time).
Cycling light sources for the cameras of cell phones and other small handheld devices generate a significant amount of heat in a relatively short period of time (on the order of less than 1 second, more typically less than 500 millisecond (ms)), which provides a limiting factor in the operation of the camera and flash. Significant delays can be encountered between flashes (the amount of time between consecutive flashes is referred to as the recovery time for the light source), due to overheating issues. Indeed, recovery times of greater than 5 seconds are not uncommon, where recovery times as short as possible are the goal of the industry. Furthermore, in a continuous lighting, or so called “torch” mode of operation, illumination levels are again limited due to overheating considerations. The possible solutions to these overheating issues are constrained by the very limited space availability within the handheld device.
In the broader field of larger electronic devices, various heat spreader technologies have been developed. So called “thermal boards” are being developed where a layer of a heat spreading material such as copper or aluminum and alloys thereof is laminated with the dielectric material, on the surface opposite or in layers opposing that of the circuit and heat-generating components, to act as a heat spreader for the heat generated from the electronic components. It is important that the heat spreader be located such that at least one layer of dielectric material separates the heat spreader from the circuit(s), since the heat spreader materials are typically electrically conductive, and would interfere with the operation of the circuits if they were in contact.
There are several commercially available “thermal boards,” sometimes called metal core printed circuit boards (MCPCB), such as Insulated Metal Substrate™ thermal boards from The Bergquist Company, T-Clad™ thermal boards from Thermagon, HITT Plate boards from Denka, and Anotherm™ boards from TT Electronics. These thermal boards utilize thermally conductive dielectric layers, either through filling the dielectric layer with thermally conductive particles as in the case of the first three, or as in the case of the Anotherm solution, through a thin anodization layer on top of the aluminum heat spreader layer. The use of thermally conductive particles can be expensive, however, and the subsequent layer must be thick enough to ensure it is pin-hole free, adding to thermal resistance in the design. Additional limitations of this approach arise from the lack of flexibility to fabricate bent or non-planar circuit structures, and the fact that the dielectric material covers the entire surface of the heat spreader layer. The use of anodization as the dielectric layer attempts to overcome some of these issues, but forces the use of aluminum as its heat spreader layer, since copper cannot be anodized. Since the thermal conductivity of aluminum is significantly less than that of copper, this can be a thermal disadvantage. All of the foregoing approaches, however, can suffer soldering difficulties, since the same heat dissipation properties that are useful during the operation of the printed circuit board and components, inhibit an assembly process that requires point sources of heat for soldering (such as hot bar bonding, for example).
To overcome some, but not all of these issues, traditional printed circuit boards can be married to a separate metal heat spreader layer in a separate process. In this arrangement, the printed circuit board can be designed with thermal vias (typically drilled holes that are plated with copper) to conduct heat better through the unfilled dielectric layer of the printed circuit board, but these may only be used in applications where electrical isolation from component to component is not required.
Moreover, traditional heat spreading materials like copper or aluminum also add significant weight to the board, which is undesirable, and the coefficient of thermal expansion (CTE) of these materials may not closely match that of the glass fiber laminate, leading to physical stress on the printed circuit board with the application of heat and, potentially, delamination or cracking.
Additionally, since the heat spreader layer on these boards is comprised of an isotropic, thin (relative to its length and width) metal material, heat tends to flow through the thickness of the heat spreader readily, and resulting hot-spots can occur in the location directly opposite the heat source.
As noted, another type of circuit assembly, referred to in the industry as a “flex circuit,” provides similar heat management problems. Flex circuits are formed by providing a circuit, such as a copper circuit as described above, on the surface of a polymer material, such as a polyimide or polyester, which functions as the dielectric layer. As the name suggests, these circuit materials are flexible and can even be provided as rolls of circuit materials that can later be married to a heat spreader layer like copper or aluminum. While very thin, the dielectric layer in flex circuits still adds appreciably to the thermal resistance in a given design, and suffers from some of the same issues observed in printed circuit boards. The use of thermal vias is still limited to electrically isolating applications as described previously. And as is apparent, the use of relatively thick and rigid metallic layers, such as of copper or aluminum, does not allow one to take advantage of the flexibility of flex circuits, where such a characteristic is important in an end-use application.
The use of a heat spreader formed of sheet(s) of compressed particles of exfoliated graphite or a graphitized polymer film (such as described in U.S. Pat. No. 5,091,025, the disclosure of which is incorporated herein by reference) can remedy many of the disadvantages encountered with the use of copper or aluminum heat spreaders, since such graphite materials provide the advantage of an 80% weight reduction compared to copper, while being able to match or even exceed the thermal conductivity of copper in the in-plane direction needed for heat spreading across the surface of a printed circuit board.
Laminates in which one or more of the layers consist of graphite sheets are known in the art. These structures find utility, for example, in gasket manufacture. See U.S. Pat. No. 4,961,991 to Howard. Howard discloses various laminate structures which contain metal or plastic sheets, bonded between sheets of compressed particles of exfoliated graphite. Howard discloses that such structures can be prepared by cold-working a graphite sheet on both sides of a metal net and then press-adhering the graphite to the metal net. Howard also discloses placing a polymer resin coated cloth between two sheets of graphite while heating to a temperature sufficient to soften the polymer resin, thereby bonding the polymer resin coated cloth between the two sheets of graphite to produce a graphite laminate. Similarly, Hirschvogel, U.S. Pat. No. 5,509,993, discloses graphite/metal laminates prepared by a process which involves as a first step applying a surface active agent to one of the surfaces to be bonded. Mercuri, U.S. Pat. No. 5,192,605, also forms laminates from sheets of compressed particles of exfoliated graphite bonded to a core material which may be metal, fiberglass or carbon. Mercuri deposits and then cures a coating of an epoxy resin and particles of a thermoplastic agent on the core material before feeding core material and graphite through calender rolls to form the laminate.
In addition to their utility in gasket materials, graphite laminates also find utility as heat transfer or cooling apparatus. The use of various solid structures as heat transporters is known in the art. For example, Banks, U.S. Pat. Nos. 5,316,080 and 5,224,030 discloses the utility of diamonds and gas-derived graphite fibers, joined with a suitable binder, as heat transfer devices. Such devices are employed to passively conduct heat from a source, such as a semiconductor, to a heat sink.
As noted, the graphite material preferred for use as the heat spreader material of this invention is sheets of compressed particles of exfoliated graphite, or a graphite film formed by the graphitization of a polymer material.
Natural graphite, on a microscopic scale, is made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially-flat, parallel, equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly-ordered graphite materials consist of crystallites of considerable size, the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites, by definition, possess anisotropic structures and thus exhibit or possess many characteristics that are highly directional, e.g., thermal and electrical conductivity and fluid diffusion.
Briefly, natural graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to with, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the “c” direction. The graphites suitable for manufacturing sheets of compressed particles of exfoliated graphite possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be chemically treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Natural graphite flake which has been chemically or thermally expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension, can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, or the like. The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy to thermal and electrical conductivity and fluid diffusion, somewhat less, but comparable to the natural graphite starting material due to orientation of the expanded graphite particles substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll processing. Sheet material thus produced has excellent flexibility, good strength and a very high degree or orientation. There is a need for processing that more fully takes advantage of these properties.
Briefly, the process of producing flexible, binderless anisotropic expanded natural graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a “c” direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance will, once compressed, maintain the compression set and alignment with the opposed major surfaces of the sheet. Properties of the sheets may be altered by coatings and/or the addition of binders or additives prior to the compression step. See U.S. Pat. No. 3,404,061 to Shane, et al. The density and thickness of the sheet material can be varied by controlling the degree of compression.
Lower densities are advantageous where surface detail requires embossing or molding, and lower densities aid in achieving good detail. However, higher in-plane strength and thermal conductivity are generally favored by more dense sheets. Typically, the density of the sheet material will be within the range of from about 0.04 g/cm3 to about 1.4 g/cm3.
Natural graphite sheet material made as described above typically exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll-pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the “c” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal properties of the sheet are very different, by orders of magnitude typically, for the “c” and “a” directions.
In the formation of graphitized polymer films, graphite films with high crystallinity are created by the solid-state carbonization of an polymeric film such as an aromatic polyimide film or a polymellitimide film, followed by a high temperature heat treatment to create films that are carbonized without any change in shape (though a significant shrinkage does occur), and also because the carbon films can be converted to graphite through a subsequent heat treatment in an inert atmosphere.
Polyimides have been developed for use as thermoresistant polymers in a variety of electronic applications. As such, a variety of polyimide films have been produced with different molecular structures to better fit the properties of the film to the specific application. This fact allows for differing properties of the resultant graphite film as the quality and composition of the polyimide film combined with the graphitization techniques control the resultant graphitic properties.
A film such as a polyimide film is first cut and shaped to anticipate the subsequent shrinkage during the carbonization step. During carbonization a large amount of carbon monoxide may evolve from the film accompanied by a substantial shrinkage of the film. The carbonization may take place as a two step process, the first step at a substantially lower temperature than the second step. During the first step of carbonizing a polyimide film, the weight loss is primarily due to the breakage at the carbonyl groups in the imide part of the polyimide film. Specifically, the ether oxygen appears to be lost at the end of the first step. In the second step of carbonization, nitrogen gas may be released during the decomposition of the imide groups of the film.
The graphitization process includes a high temperature heat treatment with the temperature of the heat treatment resulting in different alignment of the carbon atoms. Specifically, dependent upon the selected film, pores exist between the carbon layer stacks after graphitization at certain temperatures. For example, at 2450° C., a polyimide film, after the graphitization step, may still be turbostratic as flattened pores are oriented between the carbon layers. Conversely, at 2500° C., the same film would have the pores collapse resulting in a graphitic film with virtually perfect carbon layers.